The isomer distribution of C6H6 products from the propargyl radical gas-phase recombination investigated by threshold-photoelectron spectroscopy

The resonance-stabilization of the propargyl radical (C3H3) makes it among the most important reactive intermediates in extreme environments and grants it a long enough lifetime to recombine in both terrestrial combustion media and cold molecular clouds in space. This makes the propargyl self-reaction a pivotal step in the formation of benzene, the first aromatic ring, to eventually lead to polycyclic aromatic hydrocarbons in a variety of environments. In this work, by producing propargyl radicals in a flow tube where propyne reacted with F atoms and probing the reaction products by mass-selected threshold-photoelectron spectroscopy (TPES), we identified eight C6H6 products in total, including benzene. On top of providing the first comprehensive measurements of the branching ratios of the eight identified C6H6 isomers in the propargyl self reaction products (4 mbar, 298 K conditions), this study also highlights the advantages and disadvantages of using isomer-selective TPES to identify and quantify reaction products.

Dear Dr Hrodmarsson, Your manuscript titled "The isomer distribution of C 6 H 6 products from the propargyl gas-phase recombination investigated by threshold-photoelectron spectroscopy" has now been seen by 3 referees.You will see from their comments below that while they find your work of interest, some important points are raised.We are interested in the possibility of publishing your study in Communications Chemistry, but would like to consider your response to these concerns in the form of a revised manuscript before we make a final decision on publication.
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Best regards, Katrin Erath-Dulitz, DPhil Editorial Board Member Communications Chemistry orcid.org/0000-0003-0489-6038Reviewers' comments: Reviewer #1 (Remarks to the Author): The propargyl self-reaction has important significance in chemical reaction kinetics.It may help in understanding the formation of PAHs in the interstellar medium (ISM).The authors of the reviewed manuscript determined the branching ratios of propargyl self-reaction by combining i2PEPICO measurements of C 6 H 6 isomers, quantum chemistry calculations, and Franck-Condon factor simulations.The branching ratios determined experimentally have been used to benchmark theoretical calculations.
Both the experimental measurements and computational results are reliable and of high quality.The objective, to accurately determine reaction branching ratios, has been achieved convincingly.The combined experimental/computational methodology developed here will have wide applications in chemical kinetic studies.Furthermore, the manuscript provides valuable insights into the complex chemistry of C 6 H 6 isomer formation from the propargyl self-reaction.Therefore, I would recommend the manuscript for publication.
It would be helpful to provide more details on the Monte-Carlo simulation in the supporting information.
The manuscript itself needs proofreading and polishing.For example: -I believe ist would be better to use "resonance-stabilization" than "resonant-stabilisation"; -In a couple of places, the authors used "… allots it …".Is it better to just say "… makes it …"?-"…understanding the fundamental reactions that lead to the formation of PAHs and eventually soot particles are of paramount interest…" "are"-->"is".
-"Experimental validation of the above-mentioned model and quantification of the benzene exit channel are challenging…" "are"-->"is".
-" … ratio determined by their NMR signature."--> " a ratio …" -"…a remarkable good agreement…" --> " … remarkably good …" Reviewer #2 (Remarks to the Author): This manuscript reports a careful study of the 78 u products observed in a low pressure room temperature flow containing propyne and F atoms in helium.The conditions are chosen to favor formation of the propargyl radicals for the study of the self-reaction, an important process leading to the formation of the first aromatic ring in combustion and astrochemical environments.The work is performed using threshold photoelectron spectroscopy at the SOLEIL synchrotron using a double imaging VMI system, and the spectra are compared to their own for isolated candidate isomers or to literature values where available, and to theory.This is certainly state-of-the art and adds significantly to the knowledge base on this important system.I have only a few minor suggestions for the authors after which I recommend publication.I suggest combining Fig. 4 a and d into a single figure with two colors to represent the two isomers.It is the only case where two different species are compared to a single spectrum and is a bit confusing the way it is shown.
It would be useful to discuss the temperature dependence of the branching further.When they say there is agreement with Klippenstein and Miller, is this at room temperature?Implications for low temperature would also be of interest for astrochemistry.
In a number of cases the English is awkward: particularly in the opening sentences, but also in a few other places (e.g."criterium").
The references almost all list only the first author.I suspect this is some default in the bibliography software and not a conscious choice and I hope it is not in keeping with the journal standard.
The author contribution statement is from the template and is thus not a contribution statement at all.
Reviewer #3 (Remarks to the Author): The authors provide an interesting demonstrative application of the use of threshold photoelectron spectroscopy (TPES) to determine the isomer-resolved product distribution for one of the most important reactions in the chemistry of hydrocarbon ring formation chemistry.The work is reasonably well described, informative, and novel.The TPES method holds promise for many further studies of product state distributions.Thus, I recommend it be accepted for publication once the authors respond to a few minor questions I have.
It is not clear to me why the authors use the calculated spectra for their fitting functions rather than the experimentally determined reference spectra.Some detailed explanation of the rationale for that choice would be helpful.I am curious to see what the spectra would have looked if they had simply employed the product distributions predicted by Miller and Klippenstein.A plot of that (perhaps in the supplementary material) could provide a very nice visual illustration of how significant the deviations are.
On line 286, the authors indicate that similar signals imply a fulvene/benzene ratio of 0.5.But on lines 282/283, they indicate that the conversion from signal to ratio instead increases the estimate by a factor of two.I am confused as to why in one case, the conversion increases the ratio, and in the other it decreases the ratio.Furthermore, in Table I, the ratio is 14/11.So, in the end, I am very confused as to what they are saying about the fulvene to benzene ratio.
My interpretation of the purpose of the Monte Carlo simulations was to obtain error bars on the estimated branching ratios.If I understand correctly, the method employed effectively considers single variations in each ratio, but not simultaneous variations in the whole set of branching ratios.If this understanding is correct, then their approach effectively explores the local minimum that they settled on by hand.Is this understanding correct?If it is, they might care to comment on whether or not there might be other local minima for the fits and how one might explore such possibilities.
There are some odd word choices in places -for example, "emblematizes", "agglutination", "fenyl".Also, the phrase "equilibrium reaction present between 2E13BD and fulvene" makes me think that somehow 2E13BD reacts with fulvene.Presumably, what is meant is that 2E13BD and fulvene equilibrate.
Reviewer #1 (Remarks to the Author): The propargyl self-reaction has important significance in chemical reaction kinetics.It may help in understanding the formation of PAHs in the interstellar medium (ISM).The authors of the reviewed manuscript determined the branching ratios of propargyl self-reaction by combining i 2 PEPICO measurements of C6H6 isomers, quantum chemistry calculations, and Franck-Condon factor simulations.The branching ratios determined experimentally have been used to benchmark theoretical calculations.
Both the experimental measurements and computational results are reliable and of high quality.The objective, to accurately determine reaction branching ratios, has been achieved convincingly.The combined experimental/computational methodology developed here will have wide applications in chemical kinetic studies.Furthermore, the manuscript provides valuable insights into the complex chemistry of C6H6 isomer formation from the propargyl selfreaction.Therefore, I would recommend the manuscript for publication.
It would be helpful to provide more details on the Monte-Carlo simulation in the supporting information.
This manuscript reports a careful study of the 78 u products observed in a low-pressure room temperature flow containing propyne and F atoms in helium.The conditions are chosen to favor formation of the propargyl radicals for the study of the self-reaction, an important process leading to the formation of the first aromatic ring in combustion and astrochemical environments.The work is performed using threshold photoelectron spectroscopy at the SOLEIL synchrotron using a double imaging VMI system, and the spectra are compared to their own for isolated candidate isomers or to literature values where available, and to theory.This is certainly state-of-the art and adds significantly to the knowledge base on this important system.I have only a few minor suggestions for the authors after which I recommend publication.
I suggest combining Fig. 4 a and d into a single figure with two colors to represent the two isomers.It is the only case where two different species are compared to a single spectrum and is a bit confusing the way it is shown.
Author response: After various trials, we find that combining the two into a single figure fails in clarifying the situation but rather makes things more complicated.We prefer to leave these figures as they are, but clarifications on reference spectra have been added in the "Reference TPES" section of the article to clear up any confusion.
It would be useful to discuss the temperature dependence of the branching further.When they say there is agreement with Klippenstein and Miller, is this at room temperature?Implications for low temperature would also be of interest for astrochemistry.
Author response: We specified that the agreement concerned the temperature of our study (room temperature).
The implications for astrochemistry (very low temperature and very low pressure) are difficult to predict without specific calculations.Indeed, Miller and Klippenstein's study only covers branching ratios for temperatures above 300K and pressures above 1 torr.Moreover, radiative association, not considered in Miller and Klippenstein study, can play an important role at very low temperatures.For Titan, Vuitton et al. 2019 (S.Klippenstein is one of the authors) use a high rate for radiative association equal to 6.5e-11 molecule -1 .cm 3 .s - for Titan temperature conditions (T = 90-150K) (reaction 169 in Table B.15 of Vuitton 2019).The study of the C3H3 + C3H3 reaction at low pressure and low temperature of the dense molecular cloud remains to be done, in particular for the competition between the radiative association and the bimolecular H + C6H5 pathway.
In a number of cases the English is awkward: particularly in the opening sentences, but also in a few other places (e.g."criterium").
The references almost all list only the first author.I suspect this is some default in the bibliography software and not a conscious choice and I hope it is not in keeping with the journal standard.
Author response: This is the standard bibliographic format for Communications in Chemistry where the author list is abridged if there are more than five authors.
The author contribution statement is from the template and is thus not a contribution statement at all.Dear Dr Hrodmarsson, Your manuscript titled "The isomer distribution of C 6 H 6 products from the propargyl gas-phase recombination investigated by threshold-photoelectron spectroscopy" has now been seen again by two of our referees, whose comments appear below.In light of their advice I am delighted to say that we are happy, in principle, to publish a suitably revised version in Communications Chemistry under the open access CC BY license (Creative Commons Attribution v4.0 International License).
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